A current trend in optical telecommunications systems to achieve high traffic capacity and spectral efficiency is to exploit multi-level modulation formats like mQAM. These formats are useful for scaling the traffic capacity up to 400 Gb/s as is being demonstrated currently and soon up to 1 Tb/s.
However the realization of transmitter for multi-level modulation formats is not straightforward and may require complex architecture involving many optical and electrical components.
Many transmitter configurations have been identified with different pros and cons. The basic building block common in all schemes is the Mach Zehnder type modulator (MZM) which has an intrinsic non-linear voltage/optical power characteristic response.
            MZM      ⁢                          ⁢      transfer      ⁢                          ⁢      function        =                  I        output            =                        1          2                ⁢                              I            0                    ⁡                      (                          1              -                              cos                ⁡                                  (                                                                                    π                        ⁢                                                                                                  ⁢                        V                                                                    V                        π                                                              +                    ϕ                                    )                                                      )                                ,
The function above shows that the MZ modulator has a sinusoidal response to the applied voltage. Other types of modulator have other characteristic distortions. This feature creates major issues in multi-level modulation schemes (e.g. mQAM) where multi-level electrical driving voltages are applied to the MZ modulator arms. The result is that the output constellation diagram is distorted according to the non-linear transfer function. In a traditional transmitter scheme a single MZM is used and a multilevel electrical signal is applied to it. In other known proposed schemes, multiple MZMs are combined in different ways, such as in series or parallel in order to build the mQAM constellation using only binary driving voltages. These solutions have the drawback of requiring several expensive optical modulators and components and complicate the transmitter scheme.
In one example a first MZM generates a QPSK scheme. A second MZM rotates it to generate a 16 QAM output. Furthermore, in these alternative schemes, it is anyway necessary to provide a very accurate bias and amplitude control of the MZMs in order to build up the QAM constellation correctly.
In the field of high capacity optical transport systems, techniques to avoid the MZM distortion problem are based on finding alternative and complex transmitter schemes involving a serial or parallel combination of several MZM modulators driven by binary signal. Another known approach is to modulate the MZM with a reduced amplitude RF signal in order to use only the linear part of the MZM dynamic range. This has the undesirable effect of reducing the extinction ratio of the output signal and thus reducing performance.
This problem of MZM non linearity, amongst other modulator distortions, is currently a limiting factor in the design of high bit rate interfaces. Currently it is addressed by over-engineering of the transmitter which increases cost, power consumption and space requirements.